With the advent of nuclear powerplants, new problems arose with respect to the welding, inspection, and repair of pipes. In addition to the need for highly accurate welds, it is often necessary to weld and inspect pipes in areas which are contaminated by dangerous levels of radioactivity. Accordingly, there arose a need for an apparatus which is capable of positioning and operating a welding head or inspection device by remote control and which can provide precision positioning of a tool with respect to a workpiece.
The broad concept of a carriage mounted to travel along an elevated track for transporting a welding head along a circumferential welding track is well known in the art. Typically, however, many self-propelled carriages known in the art suffer the disadvantage of binding in the track and of traveling more slowly as the tool carriage moves upward on the track than when it moves downward on the track, disadvantages which can result in uneven welds and increased maintenance demands.
One such circumferentially-moving apparatus is shown in U.S. Pat. No. 3,621,183, which discloses a butt-welding machine for metal sheets or large-diameter pipes. The apparatus comprises a rail or track which is mounted to one of the workpieces in parallel relation to the gap between the workpieces. A self-propelled carriage is mounted on the track for supporting and oscillating a welding head transversely with respect to the weld. As the carriage moves along the rail, the weld head is reciprocated in a direction perpendicular to the weld by means of a motor and reduction gear unit. The electrode oscillates constantly from one edge to the other of the gap which the weld head tracks, and the amplitude of oscillation is automatically controlled so that the path described by the electrode is always situated between the edges of the two sheets or pipes being joined, even if the gap between them is irregular, and even if the rail supporting the weld head carriage is not exactly parallel to the gap. However, this apparatus provides only for motion along the gap between the workpieces and for oscillation within the gap, and does not provide for movement perpendicular to the gap between the workpiece.
A different approach to positioning a tool is shown in U.S. Pat. No. 3,665,148, which discloses an apparatus utilizing electric servo motors and ball screw drives for correct positioning of a tool with respect to the workpiece. The apparatus comprises an automated six-axis manipulator for repetitively positioning a tool along the primary coordinate axes to achieve accurate work processing relationship with an article moving along an assembly line. While the use of electric servo motors and ball screw drives to position the tool is of interest, the tool manipulator disclosed in this patent is not readily adaptable to circumferential welding of a pipe.
U.S. Pat. No. 3,532,807 discloses an automatic welding apparatus with a track and moveable carriage, which uses an automatic closed circuit television arc guidance control system. However, the patent does not disclose an arrangement for achieving longitudinal motion, and it further does not address the problems peculiar to pipe welding applications.
Accordingly, there is a need in the art for an a tool positioning apparatus which provides for both circumferential and longitudinal indexing of a tool with respect to a pipe.